Bisimide resins, e.g. bismaleimide resins, are advantageously used in providing resin matrix composites, e.g. glass or carbon fiber reinforced laminates, to achieve enhanced properties such as greater thermal stability and lower moisture sensitivity than is possible with other composites, e.g. composites based on epoxy or other resins. A common bisimide, i.e. bis(4-maleimidophenyl)methane, exhibits poor solubility in many organic solvents of choice. Its use in commercial manufacture of laminates is facilitated by dissolving the bisimide in N-methyl pyrrolidone (not a preferred solvent) and by chain extension by Michael addition reaction with diamines.
Nishikawa, et al., disclose in U.S. Pat. No. 4,460,783 that certain aromatic ether bismaleimide compounds such as bis(maleimidophenoxyphenyl) propane and the like are highly soluble in desirable solvents such as acetone, toluene, methyl ethyl ketone and the like. See also Harvey et al in "New Aromatic-Ether Bismaleimide Matrix Resins", ANTEC '86, page 1311. It has been discovered that in their purer forms such aromatic ether bismaleimides have low solubility. It has been further discovered that the solubility of such aromatic ether bismaleimides, as well as other bismaleimides generally, can be enhanced by the presence of a solubilizing amount of compounds having at least one terminal maleamic acid group. Bisimide resins have been preferably prepared with low levels (if any) of terminal maleamic acid groups which tend to liberate water from ring closing imidization during cure of such resin. Such water can vaporize at commonly used fabrication temperatures, tending to generate voids or blisters in fabricated articles such as laminates., For instance, Bargain in U.S. Pat. No. 3,839,358 discloses that bismaleimides, e.g. bis(4-maleimidophenyl)methane, are prepared by reacting a diamine with maleic anhydride followed by catalytic cyclodehydration. The bismaleimide is purified, i.e. separated from acid-containing species, by washing with a basic solution, e.g. aqueous sodium bicarbonate. See also Balme who discloses in U.S. Pat. No. 3,975,401 a method for reducing the proportion of maleimido-acid (i.e. maleamic acid) groups in the precipitated product of cyclodehydration.
Although such acid groups can liberate water during cure conditions, it has been discoverd that low amounts of liberated water can be tolerated in many fabrication practices. However, in critical applications it is especially desirable that bisimide resins cure with minimal liberation of water, e.g. to avoid blisters that can form when laminates of the cured bisimide resin are subjected to high temperatures, e.g. as in vapor phase soldering.
An object of this invention is to provide soluble bismaleimide resin comprising predominately bismaleimides and low but solubilizing amounts of maleamic acids.
Another object is to provide concentrated solutions of such resins in common organic solvents. Yet another object is to provide such solutions with desirably long stability, for instance, for days, weeks or even longer.
Still another object is to provide laminates, and methods of making laminates, from such resins, especially from solutions that rapidly form a viscous thermoplastic resin that allows for minimal resin flow from reinforced matrices even during severe curing conditions, e.g. high pressure.
These and other objects of this invention are possible since it has been discovered that the addition of minor amounts of maleamic acids can greatly enhance the solubility of bismaleimides in common organic solvents, affording greatly enhanced utility in fabrication, e.g. of laminates, without substantially deleterious effects of liberated water.